Effect of hyperglycemia on D-glucose transport across the brush-border and basolateral membrane of rat small intestine.

Experimental hyperglycemia leads to an increase in the capacity of the rat small intestine to absorb glucose. This effect occurs within hours from the onset of hyperglycemia and is thought to involve an induction of glucose transport in the brush-border and/or basolateral membrane of the intestinal epithelium. We devised a protocol for the simultaneous preparation of brush-border vesicles and basolateral vesicles from rat small intestine to determine the locus for the induction of glucose transporter in hyperglycemic rats. A 6 h period of intravenous infusion with a 30% glucose solution had no effect on the initial rate of glucose uptake across jejunal or ileal brush-border vesicles when measured in the absence of a Na+ gradient, suggesting that enhanced glucose uptake is not dependent on an increase in the number of Na+-dependent secondary active glucose transporters in the brush-border. Hyperglycemia did not effect the rate of glucose uptake across ileal basolateral vesicles but did cause a 78% increase in the initial rate of carrier-mediated D-glucose uptake across jejunal basolateral vesicles. The induction of glucose transport in the jejunal basolateral membrane was characterized by a rapid rate of glucose equilibration across the vesicles (t 1/2 = 46 s sorbitol infused controls, 18 s hyperglycemia) and a 75% increase in the Vmax for carrier-mediated glucose uptake with no significant change in Kt. When the rats were pretreated with cycloheximide prior to intravenous infusion, the initial rate of D-glucose uptake dropped to 13% of that seen in jejunal basolateral vesicles prepared from untreated rats. These results suggest a rapid turnover rate for the Na+-independent glucose transporter in the basolateral membrane of the enterocyte. An increase in the number of functioning glucose transporters in the basolateral membrane may play an important role in the short-term induction of glucose absorption by the jejunum of the hyperglycemic animal.